Eben Fiske Ostby, owner and descendant of the Colonial family that has held the property for 360 years, donated 141 acres to the nonprofit Sylvester Manor Educational Farm on Monday.

Included in the donation are the Manor House, grounds, the 1810 windmill and barns.

The gift is the largest in Island history, and according to the Manor’s Executive Director Cara Loriz, “ranks among the most significant outright gifts of land and cultural assets on Long Island.”

This landmark event comes in the wake of Mr. Ostby’s donation two years ago of 83 acres, which are preserved as farmland through the town, Suffolk County and the federal government.

The nonprofit now takes control of the acreage — valued at $12.3 million — that includes waterfront property along Winthrop Road. According to Ms. Loriz, if Mr. Ostby had not decided to give the property to the nonprofit, close to 100 houses could have been built on the Manor grounds..

Mr. Ostby will retain about 12 acres of woodland property.

The owner also gave Sylvester Manor’s long-time caretaker, Gunnar Wissemann, a cottage he and his family have lived in for more than 20 years, subject to a ground lease for the land beneath it, which is now owned by the nonprofit.

Supervisor Jim Dougherty praised the gift of the 141-acre parcel, comparing it to the Nature Conservancy’s 1980 acquisition of Mashomack.

“On behalf of all Shelter Islanders, I want to thank Eben Ostby for his generous and invaluable contribution to Shelter Island’s future,” Mr. Dougherty said.